Polyurethanes first attained substantial commercial importance in the production of flexible and rigid foams. As late as 1968 it was estimated, Encyclopedia of Polymer Science and Technology, Interscience Division of John Wiley & Sons, Inc., New York (1969), Volume 11, pages 508, that of a total production in the United States of approximately 500 million pounds of finished polyurethane polymer, "The major portion of this market is still in flexible and rigid foams; comparatively speaking all other areas are still relatively small, consisting of five to ten percent of the total market." However during the time of major growth in the foam industry, other areas of use and application, including that of surface coatings, were investigated quite extensively and had appreciable growth. In the surface coating fluid, the commercial systems initially developed, in general, involved the use of either solvent-based, reactive high solids prepolymers or fully reacted urethane lacquers. The development of commercially acceptable polyurethane latices, of commercial significance, has largely occured since 1960 and a number of methods are presently used to produce polyurethane latices. The two most important methods, presently used, to produce polyurethane emulsions, are believed to be:
A. Direct Emulsification -- where substantial amounts of surfactants are used to emulsify a prepolymer, with high speed agitation; and
B. Emulsification by Salt Formation -- where groups (usually secondary amino groups) are built into the prepolymer which are subsequently converted into salts having surfactant properties during emulsification.
Both of these techniques suffer from certain disadvantages. In the case of direct emulsification, it has been found necessary to use a substantial amount of surfactant, usually 4 - 6 parts of surfactant 100 parts of prepolymer, in order to obtain a stable emulsion and it has been suggested, the Applications of Synthetic Resin Emulsions by H. Warson, Ernest Benn, Ltd., London (1972), page 271, that in general such emulsions should be applied within 24 hours of preparation. When such emulsions (latices) are applied as a coating, the surfactant remains in the final film and results in increased water sensitivity of the film and reduced physical properties. The latices produced by the salt formation technique have particles of large size 2-5 microns or larger, with most of the particles being in the range of 2-3 microns, H. Warson, supra, page 270; with the result that the latices are not stable and a considerable percentage settles out of the latex on standing.